The interaction between fibrous rabbit muscle actin and four globular proteins in solutions of physiological ionic strength has been studied by means of sedimentation equilibrium and viscosity. Techniques for optimization of experimental protocols and data analysis in the quantitative characterization of macromolecular hetero-associations via measurement of sedimentation equilibrium have been carried out via simulation studies. The effect of high protein concentration upon the sedimentation coefficient of serum albumin has been carefully measured as a function of pH and ionic strength. Fully automated scanner control capability has been added to a custom-designed and built PC-based data acquisition system for the Beckman Model E analytical ultracentrifuge.